Method for operating a shoe press, shoe press, machine comprising a shoe press, and use of an ink in a lubricant for a shoe press

ABSTRACT

A shoe press has a shoe on which a radial inner surface of a press jacket slides. In a method of operating the shoe press, lubricating fluid is introduced between the shoe and the press jacket for lubricating purposes. The press jacket has at least one polymer layer into which a reinforcing structure is integrated, the reinforcing structure including at least one reinforcing thread. The lubricating fluid, the at least one polymer layer, and/or the at least one reinforcing thread is/are designed such that the lubricating fluid reaches the at least one reinforcing thread of the reinforcing structure and produces a color change in same in the event the polymer layer breaks. There is also described a shoe press, the inclusion of a press jacket in such a shoe press, and a machine with such a shoe press.

The invention is based on a press cover, in particular for a press device for treating a fibrous web, for example to smooth or dewater the latter, in detail as claimed in the independent claims. The invention also relates to a shoe press and the use of a press cover in such a shoe press and also a machine comprising such a shoe press, in detail as claimed in the subordinate claims.

Press devices such as shoe presses have for a long time been a constituent part of modern paper machines. They substantially comprise a shoe (also called a press shoe) which is arranged in a stationary manner and extends in a cross-machine direction, and a press cover revolving around the stationary shoe. Said press cover is deformable and assumes substantially a tubular form in operation. The shoe is shaped in such a way that it forms a press nip (press gap) with an opposing roll. The press nip is defined by the contact surface of the opposing roll in the shoe. The shoe is designed to be movable and can be moved onto the opposing roll.

Enormous requirements in relation to its stability are placed on the press cover, specifically with regard to surface hardness, resistance to pressure, temperature and hydrolysis. The press cover is additionally subjected to high alternating bending loads during operation. As it runs in at the edge of the shoe—before the press nip as seen in the direction of rotation of the press cover—bending is firstly carried out over a comparatively small radius. This changes immediately into an opposite bending as it passes through the press nip. As it runs out at the other edge of the shoe, that is to say—after the press nip as seen in the direction of rotation of the press cover—opposite bending takes place again. This deformation of the press cover as it runs in and out is also designated a changeover nip. It is easy to see that the tendency of the press cover to break, particularly at this point, is very high as a result of the high mechanical stress. Accordingly, the prior art discloses many measures which are intended to increase the stability of the press cover.

The press cover must thus be sufficiently flexible in order that it can be led around the shoe, it must be sufficiently rigid in order that it is not deformed or compressed too highly under the press load in the nip, and it must be sufficiently wear-resistant. Press covers therefore consist of a single-layer or multi-layer polymer layer, preferably of polyurethane, in which reinforcing threads in the form of laid or woven fabrics can be embedded.

The present invention relates to generic objects of this type mentioned at the beginning.

Press covers disclosed by the prior art tend to premature failure during proper use as a result of—often only local—overloading in the nip. Such overloading arises when, during what is known as a lump passage, a foreign body goes through the nip. Such overloading often leads to the reinforcing threads or the polymer layer in which they are embedded tearing. A press cover which, for example, is oil-lubricated from inside can become leaky, so that oil comes into contact with the fibrous web to be produced. The press cover must be changed as a result. In practice, this leads to unplanned stoppages of the press device and therefore to increased, costly outages.

It is accordingly an object of the invention to specify a press cover which avoids the disadvantages of the prior art. In particular, total failure of the press cover as a result of even only local damage to the same as a result of overloading in proper operation is to be prevented and its consequences detected in good time. In particular, the stoppage times of a press device equipped with such a press cover are to be reduced.

The object is achieved by the features of the independent claims. Particularly preferred and advantageous embodiments of the invention are reproduced in the sub-claims.

The inventor has recognized that local overloading which has already occurred can be detected better on a press cover if the lubricating fluid, the reinforcing fibers or both the lubricating fluid and the reinforcing fibers are matched to each other or set up in such a way that the reinforcing fibers change color in the event of a breakage of the polymer layer in which the reinforcing fibers are arranged. In the event of breakage of the polymer layer, this takes place as a result of the fact that the reinforcing fibers then come into contact with the lubricating fluid. Starting from this contact, for example, a chemical reaction takes place between the two partners, which results in the color change. It is conceivable that, for example, the reinforcing fibers are coated with a dye or contain such a dye which leads to a (local) color change of the reinforcing fibers only upon contact with the lubricating fluid. Alternatively or additionally, the dye can be contained in the lubricating fluid, for example by being mixed with the lubricating fluid. Irrespective of the way in which the color change takes place, this can be a permanent color change.

The dye can be a dye which reacts with luminescence. Luminescence means that energy supplied to the dye from outside is not wholly or partly converted into thermal energy but changes the dye to an excited state and emits light (including radiation outside the range visible to the human eye). Luminescence can be subdivided into two areas, namely fluorescence and phosphorescence.

Fluorescence is the spontaneous emission of light shortly after the excitation of a material by electron transfer. Here, the emitted light generally has lower energy than that previously absorbed. Phosphorescence, on the other hand, is the property of a substance to re-emit light in the dark following irradiation with (visible or UV) light. The dye that can be added to the lubricating fluid can be both a luminescent, that is to say a fluorescent or phosphorescent, dye.

The press cover can therefore be illuminated or transilluminated with light, for example from an LED lamp or a laser. The light can be light that is not visible to the human eye, e.g. ultraviolet light. The dye can be chosen such that it exhibits luminescence, that is to say, as a result of the irradiation with the light that is not visible to the human eye, just emits light that is in turn visible to the human eye.

Possible local overloading of the press cover can then be detected optically by irradiating the press cover with the light, for example with the machine at creep speed, at which the press cover rotates more slowly than in proper operation, in which the machine produces the fibrous web. Then, an impending loss of lubricating fluid from the press cover can be avoided in good time by an appropriate exchange of the press cover. The energy supplied from outside, for example in the form of light, can be provided within the radial extent of the press cover (that is to say from the inside) or outside this radial extent (that is to say from outside on the press cover). The radial extent is in each case in relation to the longitudinal axis of the press cover. Accordingly, the energy source can be arranged inside or outside the press cover.

In principle, that stated in relation to the addition of the dye into the lubricating fluid also applies if an appropriate dye for changing the color of the at least one polymer layer or reinforcing structure is not added to the lubricating fluid itself but to the polymer layer or the reinforcing threads.

The lubricating fluid can be oil, such as hydraulic oil.

It is advantageous if there is a sufficiently high optical contrast between the reinforcing structure and the at least one polymer layer in which this is embedded. This is because if the overload is sufficiently high, this normally leads to local damage to the reinforcing structure, for example in the form of breakages of the threads. Broken threads can then be detected better by the human eye, so that a planned, premature change of the press cover can be carried out. Specifically even before the press cover becomes leaky and lubricating fluid reaches the fibrous web to be produced.

In order to achieve such a sufficiently high optical contrast, the materials of the reinforcing structure and of the at least one polymer layer can be chosen such that their transmittance levels for visible light differ from each other. If the transmittance of the material of the at least one polymer layer is chosen to be higher than that of the reinforcing structure, then the reinforcing structure can shine through the polymer layer under appropriate lighting conditions. One could also say that the material of the at least polymer layer is more transparent or more translucent (therefore less opaque) than that of the reinforcing structure.

Alternatively, such a sufficient optical contrast can be produced if the materials of the reinforcing structure and the at least one polymer layer are chosen such that the transmittance of the overall press cover for visible light is between 15% and 90%, preferably between 15% and 35%.

In optics, the transmittance describes the proportion of the incident radiant flux or luminous flux which passes completely through a transparent component. The incident radiant flux which strikes a component can be transmitted, reflected and absorbed by the latter. Accordingly, the following output balance of the radiant flux striking the component results in a transmittance (T), a reflectance (R) and an absorbance (A)

R+A+T=1

which are divided up in accordance with the material properties of the component. The transmittance therefore corresponds to that part of the incident radiant flux which is reduced by the reflectance (R) and the absorbance (A). Another designation for the transmittance is the term overall transmission T_(t). The last-named or the transmittance corresponds to the relation of the reciprocal of the incident radiant flux T₁ to the radiant flux T₂ let through the transparent component, therefore T₂/T₁. If, therefore, mention is made of the fact that, for example, the material of the at least one polymer layer has a transmittance of 50%, then this allows only half the incident radiant flux through. The transmittance or the overall transmission are defined and measurable in accordance with ASTM D 1003-00. The measurement can be carried out at any desired point on the press cover, for example including at its axial edges, that is to say, for example, in the region of the lugs by means of which such a press cover is held on the two lateral tensioning discs. The statement of the transmittance can relate to a factory-fresh, that is to say ready-to-use, press cover.

The two definitions express the fact that the press cover is sufficiently transparent to see even better the color change of the reinforcing threads in the event of a breakage of the at least one polymer layer in which the reinforcing structure is embedded. It would also be possible to say that the one polymer layer or the press cover is intrinsically uncolored.

In the sense of the invention, a press cover is understood to be a closed belt, hose or a cover which is endless about its longitudinal axis in the circumferential direction and which, as illustrated, is led through the nip (press nip) of a shoe press together with a fibrous web. To dewater the fibrous web in proper operation, the radially outermost surface (polymer layer) of the press cover can come into contact with a press felt, by which the fibrous web to be dewatered is carried directly. Depending on the embodiment of the press equipment, for example for smoothing the latter, the press cover can also come directly into contact with the fibrous web in proper operation. At its axial ends—as seen in the width direction (along the longitudinal axis)—it is open. Therefore, at its two axial ends, the press cover can be held by two lateral tensioning discs in order to form the shoe press roll. Instead of the guidance by the two lateral tensioning discs, the press cover can be guided over the press shoe and a plurality of guide rolls, as is the case in open shoe presses. Irrespective of whether the press cover is guided by the tensioning discs or the guide rolls, the press shoe (and/or the guide rolls) come into contact (temporarily) with a part of the radially innermost surface of the press cover. The radially outermost surface of such a press cover, that is to say, for example, the radially outermost polymer layer of the same, can be provided with grooves and/or blind drilled holes.

The longitudinal direction means that direction which extends parallel to the longitudinal axis of the press cover. The longitudinal axis at the same time corresponds to the axis of symmetry or rotation of the finished press cover and the press roll. The circumferential direction of the press cover extends around the longitudinal axis, as viewed around its radial limit. The term parallel also includes those angular deviations of +/−5° of two reinforcing threads lying in different planes.

The press cover or the at least one polymer layer can be produced partly or wholly from a polymer. The polymer used can be a castable, curable, preferably elastomeric polymer such as polyurethane. Consequently, the polymer can be set as a cast elastomer.

Polymer layer means a layer which comprises such a castable, curable, preferably elastomeric polymer or is produced entirely therefrom. Preferably, the polymer layer can be a cured layer produced in one piece by primary molding. In other words, this is primarily molded monolithically, that is to say produced by casting, for example. The term one-piece also includes cases in which the one layer has in turn been produced from a plurality of layers of the same material during the casting of the polymer. However, this is only to the extent to which these layers are substantially no longer visible following the curing but result in a single, preferably uniform, layer. The same is appropriately true of the finished press cover.

When a plurality of polymer layers is provided, as viewed in the radial direction, these can be arranged—at least in some sections over the width of the press cover—above one another. At least in some sections over the width of the press cover means that the press cover only has a single layer, for example at its axial ends along the longitudinal axis of the press cover, whereas it is formed with two or more layers between the axial ends. The polymer layers can, however, also extend over the entire width of the press cover. In addition, the thickness of the press cover—and thus the thickness of the individual polymer layers—can vary in some sections along the longitudinal axis in a section through its longitudinal axis. Thus, for example, the radially outermost polymer layer can be lower in the region of the width edges of the press cover, than in the center of the press cover. In other words, in the region of the width edges, the radially outermost polymer layer can be less thick than a radially inner or radially innermost polymer layer. Preferably, exactly one, two or three polymer layers is/are provided. With regard to their polymer, these can be designed identically or vary with regard to their hardness or stoichiometry of the pre-polymer. A total thickness of the finished press cover, measured in a section through the longitudinal axis of the same in the radial direction, can be 5 to 10 mm, preferably 5 to 7, particularly preferably 5 to 6 mm. According to the invention, when a single layer is provided the press cover can be produced from only one casting, i.e. monolithically, so that the single layer has the thickness just mentioned.

A finished press cover in the sense of the invention is one of which at least one polymer layer is cured and possibly finally machined, that is to say is ready for use for the purpose mentioned at the beginning in, for example, a shoe press. In a similar way, a finished polymer layer means a layer which is cured.

In the sense of the invention, a reinforcing thread is understood to be a flexible, textile linear structure which has a dominant extent and a uniformity in its longitudinal direction. If mention is made of fibers, then a single, endless fiber of a monofilament type is meant. If, on the other hand, mention is made of a fiber bundle in the sense of the invention, this does not involve monofilaments but, for its part, an individual thread such as a twist or yarn, that is to say a bundle of endless fibers or monofilaments. The fiber bundle itself can entirely possibly be produced from fibers twisted with one another.

The definition that at least the longitudinal threads are produced as reinforcing threads according to the invention means that only the longitudinal threads are so designed or, in addition, the longitudinal threads and at least one further circumferential thread are so produced. If, preferably, for example, a laid fabric made of circumferential and longitudinal threads is present, then this means that at least the longitudinal threads are designed according to the invention.

In the sense of the invention, the term reinforcing structure means a reinforcement of the at least one layer containing the polymer or consisting of the latter—that is to say the polymer layer. Here, the reinforcing structure can be embedded completely in the polymer layer, so that the reinforcing structure does not go beyond the limit of the polymer layer. In other words, the polymer layer performs the role of a matrix, which surrounds the reinforcing structure and bonds to the matrix as a result of adhesive or cohesion forces. Such a reinforcing structure can comprise textile linear structures—e.g. yarns or twists—and/or textile surface structures—such as, for example, woven fabrics, warp-knitted fabrics, weft-knitted fabrics, mesh fabrics or laid fabrics—and be producible from an appropriate starting material, for example by winding. In other words, an individual reinforcing thread according to the invention, viewed on its own, is a textile linear structure. A plurality of such reinforcing threads can thus be designed, for example, as longitudinal and/or circumferential threads such that together they form a textile surface structure. The at least one reinforcing thread which is embedded in the at least one polymer layer then represents the reinforcing structure of the press cover or the polymer layer of the latter. Starting material is understood to be that material or semifinished product by means of which the reinforcing structure of the finished press cover according to the invention is produced, that is to say the at least one reinforcing thread in the present case.

The reinforcing thread or the reinforcing structure can be produced from a polymer or comprise such a polymer. Recommended polymers are polyesters, polyethylene naphthalates or polyamides, such as aramids. Therefore, the materials of the at least one polymer layer and that of the at least one reinforcing thread or the reinforcing structure embedded therein differ.

In the sense of the invention, a press device means, for example, a shoe press, for example for dewatering or treating, such as smoothing, a fibrous web. The shoe press comprises a shoe press roll and an opposing roll which, together, form or delimit a press nip. The shoe press roll further comprises a revolving press cover and a stationary pressing element, the so-called press shoe. The latter is supported on a loadbearing, likewise stationary, yoke—for example via hydraulic press elements—and is pressed onto the revolving press cover. The press cover revolves relative to the stationary press shoe and yoke and, as a result, is pressed onto the opposing roll in the press nip. The press shoe and yoke are arranged radially within the press cover. The term stationary is understood to mean that the pressing element does not revolve relative to the shoe press roll or the opposing roll but can move translationally—onto the opposing roll and away from the latter, preferably in the radial direction of the latter—and therefore relative to the opposing roll. In addition to the fibrous web and the press cover, one or more press felts circulating endlessly in the circumferential direction and/or further endlessly circulating press belts can be guided through the press nip of the shoe press. Such a shoe press can of course comprise more than one press nip.

In the sense of the invention, a fibrous web is understood to be a laid or tangled fabric of fibers such as wood fibers, plastic fibers, glass fibers, carbon fibers, additional materials, additives or the like. Thus, the fibrous web can be formed, for example, as a paper, board or tissue web. It can substantially comprise wood fibers; small quantities of other fibers or else additional materials and additives can be present. Depending on the application, this is left up to those skilled in the art.

If, preferably, a plurality of reinforcing threads as longitudinal threads and at least one reinforcing thread as a circumferential thread which surrounds the longitudinal threads in the circumferential direction are embedded in the polymer layer as a laid fabric, then the advantages according to the invention are particularly well fulfilled. This is because a laid fabric is capable of absorbing local overloads particularly well.

The advantages according to the invention are achieved particularly well if the press cover is built up from preferably a plurality of polymer layers arranged one above another in the radial direction. If two polymer layers are provided, then the radially inner is that with the reinforcing structure according to the invention. This means that the reinforcing structure is arranged only in the radially innermost polymer layer. If three or more polymer layers are provided, then the reinforcing structure is preferably arranged in the second lowest polymer layer, that is to say in that which lies radially above the radially innermost polymer layer.

The invention also relates to a machine, such as a paper machine for producing or finishing a fibrous web, comprising a shoe press for dewatering the fibrous web.

The invention also relates to a shoe press for dewatering a fibrous web, preferably a paper, board, tissue or pulp web, comprising a press roll and an opposing roll which together form or delimit a nip, wherein the press roll comprises a revolving press cover, the press cover being formed according to the invention.

Finally, the invention relates to the use of a dye according to the invention in a lubricating fluid of a shoe press comprising a press cover for dewatering a fibrous web, preferably a paper, board, tissue or pulp web.

The invention will be explained in more detail below with reference to the drawings without restricting the generality. In the drawings:

FIG. 1 shows a partly sectioned schematic side view of a shoe press having a press cover according to an exemplary embodiment of the present invention;

FIGS. 2a and 2b show embodiments of a press cover, in each case viewed in a section through its longitudinal axis;

FIG. 3 shows a highly schematic illustration of a device for producing the press cover in a side view.

FIG. 1 illustrates a partly sectioned schematic side view of a shoe press 10 which, in the present case, comprises a press roll according to the invention such as a shoe press roll 12, and an opposing roll 14. The shoe press roll 12 and the opposing roll 14 are arranged parallel to each other with respect to their longitudinal axes. Together, they form a nip 22 or delimit such a nip.

While the opposing roll 14 here comprises a cylindrically configured roll rotating about its longitudinal axis, the shoe press roll 12 is composed of a shoe 16, a stationary yoke 18 carrying the latter and a press cover 20. The shoe 16 and yoke 18 are arranged to be stationary in relation to the opposing roll 14 and the press cover 20. This means that they do not rotate. The shoe 16 is supported by the yoke 18 and pressed by hydraulic pressing elements, not illustrated, onto the radially innermost surface of the press cover 20 revolving relative thereto. The press cover 20 surrounds the shoe 16 and yoke 18 in the circumferential direction and rotates about its longitudinal axis in the opposite direction of rotation to the opposing roll 14. Because of the concave configuration of the shoe 16 on its side facing the opposing roll 14, the result is a comparatively long nip 22.

The shoe press 10 is suitable in particular for dewatering fibrous webs 24. During the operation of the shoe press, a fibrous web 24 having one or two press felts 26, 26′ is guided through the press nip 22. In the present case, there are exactly two press felts 26, 26′, which pick up the fibrous web 24 between them in the manner of a sandwich. During the passage through the nip 22, a pressure is exerted indirectly on the fibrous web 24 in the nip 22 by the press felts 26, 26′. This is done by the radially outermost surface of the opposing roll 14, on the one hand, and the radially outermost surface of the press cover 20 coming directly into contact with the corresponding press felts 26, 26′. The liquid emerging from the fibrous web 24 is picked up temporarily by the press felt or felts 26, 26′ and any depressions (not illustrated) provided in the press cover surface. After leaving the nip 22, the liquid picked up by the depressions of the press cover 20 is thrown off before the press cover 20 enters the press nip 22 again. In addition, the water picked up by the press felt 26, 26′ can be removed by suction elements after leaving the press nip 22.

In a further embodiment of the invention, not illustrated in the figures, it is possible to dispense with the press felts 26, 26′. In such a case, the fibrous web 24 is directly in contact on one side with the press cover 20 and on the other side with the opposing roll 14, which together form a press nip. The opposing roll can then be designed as a heated drying cylinder.

Irrespective of the embodiment illustrated, the radially inner surface the press cover 20 slides on the shoe 16. The shoe 16 has a fluidic connection to a lubricant reservoir 28 to supply lubricating fluid. This is indicated by the dashed line. Although not illustrated, appropriate means can be provided to deliver the lubricant from the lubricant reservoir 28 to the shoe 16, e.g. pumps.

The press cover illustrated in FIG. 1 can be designed according to the invention, as illustrated in the following figures.

FIGS. 2a and 2b illustrate different embodiments of the invention in a cross section, not to scale and illustrated partially, through the longitudinal axis 20′ of the finished press cover 20. The distance of the longitudinal axis 20′ to the radially innermost surface of the corresponding polymer layer of the press cover 20 is likewise not illustrated to scale.

According to FIG. 2a , exactly two polymer layers are provided, namely a first 20.1 and a second 20.2. In the present case, the first polymer layer 20.1 is simultaneously the radially outermost polymer layer of the press cover 20. On the other hand, the second polymer layer 20.2 is simultaneously the radially innermost polymer layer of the press cover 20. The two polymer layers 20.1, 20.2 adjoin each other directly as seen in the radial direction, i.e. there is no intermediate layer between these two.

As illustrated, a reinforcing structure 20″ can be provided in the second polymer layer 20.2. In the present case, this is embedded completely in the polymer layer 20.2. This is indicated by the hatched circles, which can be textile surface structures or linear structures such as fibers. This means that the reinforcing structure 20″ does not extend beyond the limits of the polymer layer 20.2.

The reinforcing structure 20″ here comprises a plurality of reinforcing threads 21 serving as longitudinal threads 21.1. These are arranged in the longitudinal direction of the press cover 20, extending over its circumference at a distance from and parallel to one another. In addition, here at least one further reinforcing thread 21 is provided as a circumferential thread 21.2, which extends helically in the circumferential direction of the press cover, preferably within the same polymer layer 20.1, 20.2, 20.3 in which the longitudinal threads 21.1 are also arranged. The longitudinal threads 21.1 and the circumferential thread 21.2 form a laid fabric with one another, namely in such a way that the longitudinal threads 21.1 are arranged radially within the at least one circumferential thread 21.1—viewed in relation to the longitudinal axis 20′ of the press cover 20.

In the present case, the first and a second polymer layer 20.1, 20.2 are produced from a polyurethane. This is obtainable, for example, from a pre-polymer and a cross-linker. The respective pre-polymer itself is obtainable by reaction of an isocyanate with a polyol.

FIG. 2b shows a three-layer press cover in a modification relative to FIG. 2a . This comprises a—here radially outermost—first polymer layer 20.1, a radially innermost, third polymer layer 20.3 and a second polymer layer 20.2 arranged in the manner of a sandwich between these two. The arrangement—as also in the illustration of FIG. 2a —is related starting from the longitudinal axis 20′ of the press cover 20 as seen in its radial direction. In the present case, a (single) reinforcing structure 20″ is provided only in the second polymer layer 20.2. Of course, this could also be different so that, alternatively or additionally, such a reinforcing structure 20″ could also be arranged in the first polymer layer 20.1 and/or the third polymer layer 20.3. That explained in relation to the reinforcing structure 20″ in FIG. 2a applies analogously.

If, in FIG. 1, a lump then gets into the press nip, then this can lead to local loadings and break up the polymer layer in which the reinforcing structure is embedded. Breaking up therefore means that case in which the polymer layer is separated mechanically with the result that lubricant reaches the latter or the reinforcing structure. Then, lubricant can get into the reinforcing threads. According to the invention, in such a case, a color change, for example a local color change, then takes place in the reinforcing threads of the reinforcing structure. As a result, such a lump passage can now be detected sooner than previously, detected in good time with the means mentioned at the beginning. To be specific, still before leakage of the lubricant through the press cover to the outside in the direction of the fibrous web 24 occurs. In particular, in this way the stoppage times of a press device equipped with such a press cover can be reduced considerably.

It has been shown that the advantages according to the invention are implemented particularly well if the polyurethane of the at least one polymer layer 20.1, 20.2, 20.3 or all the polymer layers, as mentioned at the beginning, is chosen in such a way that it is more transparent than the material of the reinforcing structure 20″.

FIG. 3 shows a highly schematic side view of a device for producing a press cover 20 according to the invention. The device has exactly one cylindrical winding mandrel 4 in the present case, wherein, for example, a starting material 20′″ is applied helically to its radially outermost lateral surface here. After being embedded in the polymer, the starting material 20″″ forms the reinforcing structure 20″ of the finished press cover 20 according to the invention.

The illustration shows an initial stage of the production method. In the present case, to this end the one end of the starting material 20′″ is fixed to a polymer which is arranged on the outer circumference of the winding mandrel 4. Apart from the schematic illustration shown, the one end of the starting material 20′″ could also rest on or be applied directly to the winding mandrel 4 without a polymer initially being provided between the starting material 20′″ and the winding mandrel 4. The starting material 20′″ can be a textile surface structure or linear structure.

The winding mandrel 4 is rotatably mounted about its longitudinal axis 20′, which corresponds to the longitudinal axis of the press cover to be produced. Here, the longitudinal axis 20′ extends perpendicularly into the drawing plane. Via a line 5, a casting material, such as a castable, curable elastomeric polymer, e.g. polyurethane, is put from above through a casting nozzle 6 onto the radially outermost lateral surface of the winding mandrel 4 and onto the starting material 20′″. Such a casting material can be chosen in such a way, for example with regard to its pot time and viscosity, such that it does not drip off the winding mandrel 4 during the casting. During this, the winding mandrel 4 is rotated about its longitudinal axis in the direction of the arrow. At the same time as this rotation, the casting nozzle 6 is guided via a suitable guide, not further illustrated in FIG. 3, parallel to the longitudinal axis 20′ and along the latter relative to the winding mandrel 4. At the same time as the casting material is poured on, the starting material 20′″ is rolled and wound onto the rotating winding mandrel 4 to form helices. The casting material can get through the starting material 20′″ as far as the winding mandrel 4. In this example, after the curing step, the polymer forms a radially innermost and preferably elastomeric polymer layer, which corresponds to the polymer layer 20.2 of the press cover from FIG. 2a , of which only part is shown in FIG. 3.

The casting material emerging from the casting nozzle 6 in the present case is a mixture of a pre-polymer and a cross-linker. The first is provided from a pre-polymer container, not shown, in which it is stored or stirred. The pre-polymer is the reaction product of an isocyanate according to the invention and a polyol. It can be present in the pre-polymer container, for example, in the form of a pre-polymer made of the materials just mentioned.

The cross-linker can be provided in a cross-linker container.

The pre-polymer container and cross-linker container are associated with the device for producing a press cover 20. They have a fluidic connection via lines, likewise not illustrated, to a mixing chamber (not illustrated) connected upstream of the casting nozzle 6 in the flow direction. The pre-polymer-cross-linker mixture is therefore produced upstream and outside of the casting nozzle 6, that is to say mixed in the mixing chamber. Irrespective of the production of the mixture, this is then applied to the surface of the winding mandrel 4 to form the at least one polymer layer of the press cover 20.

In principle, it would be conceivable that two or more casting nozzles 6 could be provided. These could be connected via appropriate lines to separate pre-polymer and cross-linker containers, in order also to supply different polymers to the plurality of casting nozzles 6, independently of one another. The casting nozzles 6 could be then arranged at a distance from one another along the longitudinal axis of the press cover 20 in order to produce a plurality of polymer layers 20.1, 20.2, 20.3 simultaneously by simultaneous application of the polymer from the casting nozzles 6 in one pour.

By means of such a continuous casting process, which is also known as rotation casting, an endless cylindrically tubular press cover 20 intrinsically closed about its longitudinal axis 20′ is thus produced step-by-step over the width of the winding mandrel 4, the inner circumference of the press cover 20 corresponding substantially to the outer circumference of the winding mandrel 4.

In principle, it would be conceivable to wind the starting material 20′″ onto more than the one winding mandrel 4 shown in FIG. 3. For example, two winding mandrels could be provided, which could be arranged in parallel with regard to their longitudinal axes at a distance from one another. Alternatively, it would also be conceivable to also apply the polymer to the radially inner lateral surface of the winding model 4, for example in a spinning manner. Irrespective of the embodiment mentioned, the finished press cover 20 is finally taken off the at least one winding mandrel 4.

As illustrated in FIG. 3, the press cover 20 is designed according to the invention.

Although this is not illustrated in the figures, the reinforcing structure 20″ of the at least one polymer layer 20.1, 20.2 could also be built up from a plurality of starting materials 20′″ laid over one another in the radial direction and each extending in the longitudinal axial direction and in the circumferential direction of the press cover 20. 

1-15. (canceled)
 16. A method for operating a shoe press, the method comprising providing a press cover with at least one polymer layer having a reinforcing structure embedded therein, the reinforcing structure including at least one reinforcing thread, and the press cover having a radially inner surface for gliding on a shoe of the shoe press; introducing a lubricating fluid between the shoe and the press cover for lubrication; providing the lubricating fluid, the at least one polymer layer, and/or the at least one reinforcing thread such that, in the event of a breakage of the polymer layer, the lubricating fluid reaches the at least one reinforcing thread of the reinforcing structure and brings about a color change of the reinforcing thread.
 17. The method according to claim 16, which comprises adding a dye to the lubricating fluid, wherein the dye changes a color of the reinforcing threads of the reinforcing structure in the event of a breakage of the polymer layer.
 18. The method according to claim 17, which comprises choosing the dye from the group of luminescent dyes selected from the group consisting of a fluorescent dye and a phosphorescent dye and configured to emit light as a result of energy supplied from outside.
 19. The method according to claim 17, which comprises adding a luminescent dye responding to ultraviolet light.
 20. A shoe press, comprising: a press cover having at least one polymer layer and a reinforcing structure embedded therein, said reinforcing structure including at least one reinforcing thread, and said press cover having a radially inner surface; a shoe disposed to have the radially inner surface of said press cover glide thereon; a lubricant reservoir with lubricating fluid for introducing the lubricating fluid between said shoe and said press cover; wherein at least one of said lubricating fluid or said at least one reinforcing thread is configured such that, in the event of a breakage of said polymer layer, said lubricating fluid reaches said at least one reinforcing thread of said reinforcing structure and brings about a color change of said at least one reinforcing thread.
 21. The shoe press according to claim 20, which comprises dye added to said lubricating fluid, said dye being configured to change a color of the reinforcing threads of said reinforcing structure in the event of a breakage of said polymer layer.
 22. The shoe press according to claim 21, wherein the dye is a luminescent dye chosen from the group consisting of fluorescent dye and phosphorescent dye and configured to emit light as a result of energy supplied from outside.
 23. The shoe press according to claim 21, wherein the dye is a luminescent dye responding to ultraviolet light.
 24. The shoe press according to claim 20, wherein the lubricating fluid is an oil.
 25. The shoe press according to claim 20, wherein said at least one polymer layer is produced from polyurethane or comprises a polyurethane, and said at least one reinforcing thread is produced from a polymer or comprises a polymer, and said polymer is selected from the group consisting of polyester, polyethylene naphthalate, and polyamide.
 26. The shoe press according to claim 20, wherein said polymer is aramid.
 27. The shoe press according to claim 20, wherein a material of said reinforcing structure and a material of said at least one polymer layer have mutually different levels of transmittance for visible light.
 28. The shoe press according to claim 27, wherein the transmittance of the material of said at least one polymer layer is higher than the transmittance of said reinforcing structure by more than 1.1-1.5 times.
 29. The shoe press according to claim 20, wherein a material of said reinforcing structure and a material of said at least one polymer layer are chosen such that a transmittance of the entire press cover for visible light lies between 5% and 100%.
 30. The shoe press according to claim 29, wherein the transmittance of the press cover lies between 15% and 35%.
 31. The shoe press according to claim 29, wherein the transmittance of the press cover corresponds to a total transmission according to ASTM D 1003-00 and is measured in a radial direction of the press cover.
 32. The shoe press according to claim 20, wherein said at least one polymer layer is undyed.
 33. The shoe press according to claim 20, wherein said at least one reinforcing thread is one of a plurality of longitudinal reinforcing threads extending in a longitudinal direction of the press cover, over a circumference of the press cover, and at a spacing distance and parallel to one another.
 34. The shoe press according to claim 33, wherein at least one further reinforcing thread is provided as a circumferential thread that extends helically in the circumferential direction of the press cover within said polymer layer, and wherein said longitudinal reinforcing threads and said at least one further reinforcing thread that is formed as a circumferential thread together form a laid fabric, with said longitudinal reinforcing threads being arranged radially within said at least one circumferential thread, viewed in relation to the longitudinal axis of the press cover.
 35. The shoe press according to claim 20, wherein said at least one polymer layer is one of a plurality of polymer layers and said at least one polymer layer, viewed in relation to the longitudinal axis of the press cover, is a radially inner or innermost polymer layer and said plurality of polymer layers includes a radially outermost polymer layer.
 36. The shoe press according to claim 35, wherein the plurality of polymer layers are exactly two polymer layers with the radially inner polymer layer simultaneously being the radially innermost polymer layer of the press cover.
 37. A machine, comprising a shoe press according to claim
 20. 38. In a shoe press having a shoe, a press cover with a radially inner surface disposed to glide on the shoe, and a lubricant reservoir for introducing a lubricating fluid between the shoe and the press cover, the improvement which comprises: the press cover having at least one polymer layer and a reinforcing structure embedded in the polymer layer, with the reinforcing structure including at least one reinforcing thread; the lubricating fluid containing a dye; and the lubricating fluid and/or the at least one reinforcing thread being configured such that, on occasion of a breakage of the polymer layer, lubricating fluid and the dye reach the at least one reinforcing thread of the reinforcing structure and bring about a color change of the at least one reinforcing thread.
 39. The improvement according to claim 38, wherein the dye is a luminescent dye chosen from the group consisting of fluorescent dye and phosphorescent dye and configured to emit light as a result of energy supplied from outside.
 40. The improvement according to claim 38, wherein the dye is a luminescent dye responding to ultraviolet light.
 41. The improvement according to claim 38, wherein the lubricating fluid is an oil. 